The present invention relates in general to an electron microscope of the types used for measuring a dimension of a fine pattern formed on a semiconductor substrate, and, more particularly, the invention relates to a scanning electron microscope having the function of calculating measurement data associated with a dimension measuring position in a cross section of a pattern.
In semiconductor wafer manufacturing processes, finer design of multilayer thin-film patterns formed on a wafer has been advancing rapidly, and process monitoring to monitor whether or not the patterns are formed as designed is becoming increasingly important. According to the 2002 edition of the ITRS (International Technology Roadmap for Semiconductors), which shows a roadmap for finer patterns of semiconductors, it is anticipated that the wiring pattern width of a transistor gate, where the finest pattern on a semiconductor wafer is formed, will become approximate 25 nm or less in about 2007. Therefore, it is necessary to have the ability to accurately measure such a fine pattern at a semiconductor manufacturing site from now on.
Scanning electron microscopes for use in pattern width measurement (measuring SEM (Scanning Electron Microscope), or a CD (Critical Dimension) SEM) that can pick up the image of a pattern under a hundred thousand to two hundred thousand magnifications, have conventionally been employed as a dimension measurement tool for measuring the width of a fine pattern in the order of several tens of nanometers. Patent Reference 1 (JP-A No. 11-316115) describes an example of dimension measurement processing employing such a scanning electron microscope. In this example, a pattern dimension is calculated as the distance between the left and right measuring points detected in an image profile created from a part area in the picked-up image of a pattern under measurement by averaging a signal wave of the pattern in the longitudinal direction of the pattern.
In addition to this, a method for measuring pattern dimensions by verifying the image profile of a pattern to be measured against a library of image profiles of various cross-sectional shapes that have been created beforehand with an electron beam simulation is disclosed in Non-Patent Reference 2 (A Simulation Study of Repeatability and Bias in the CD-SEM. J. S. Villarrubia et al., Metrology, Inspection, and Process Control for Microlithography XVII. pp. 138).
At the time of measuring a pattern of a semiconductor, which is likely to become increasingly finer, it can be considered that the measuring position in the cross-sectional shape of the pattern will become important from now on. For example, at the time of measuring the width of the wiring pattern of a transistor gate, since the bottom dimension of the wiring pattern of the gate largely affects characteristics of the transistor, it is necessary to accurately control the width of the bottom at the manufacturing site. Thus, from now on, it is necessary to perform manufacturing control using a dimension measurement value corresponding to a measuring position in the cross-sectional shape of the pattern.
However, in employing a dimension measurement method using conventional technology, a pattern dimension is calculated as the distance between the left and right measuring points detected in an image profile created from the picked-up image of the pattern under measurement. Methods for detecting pattern edges from the image profile include a method of specifying a height position on the image profile and a method of specifying a point of a specific slope on the image profile. However, in these methods, since a measuring position is determined from only the obtained image profile information, there is no clear correlation between a measuring position detected from the image profile and a measuring position in the cross-sectional shape of the pattern.
Methods for measuring a clear measuring point in the cross-sectional shape of a pattern currently include a method of observing a cross-section of a cut semiconductor wafer with a scanning electron microscope, etc., and a method of observing with an AFM (Atomic Force Microscope). However, there is a disadvantage have in that it is costly and time-consuming to perform these methods.
Further, in the method for measuring pattern dimensions by verifying the image profile of a pattern to be measured against a library of image profiles of various cross-sectional shapes that have been created beforehand with an electron beam simulation, as disclosed in non-patent document 1, the ability to obtain a dimension corresponding to the cross-sectional shape of the pattern has not been achieved at this time. Further, measuring dimensions with the method disclosed in non-patent document 1 is based on the assumption that models of various dimensions and shapes have been prepared in large quantities, whereas there is a problem in that dimension measurement cannot be performed with high accuracy in the case of a small number of models.